The present invention relates to the preparation of amine oxides and more particularly to a method for preparing aromatic amine oxides rapidly in high yield.
The preparation of aliphatic amine oxides from tertiary amines is a known process. The preferred oxidizing agent therefor is a peroxidizing agent, such as hydrogen peroxide, typically provided in aqueous form, though peracids additionally may be used as the oxidizing agent. Relatively mild oxidation conditions for such process typically comprehends temperatures of about 60.degree.-80.degree. C. or thereabouts under sub-atmospheric, atmospheric, or super-atmospheric pressure. Reaction times can range from as short as a few hours on up to 10-15 hours or more depending upon the oxidation conditions and particular reactants used.
Unfortunately, the preparation of aromatic amine oxides from aromatic amines is not so simple a procedure as is the preparation of aliphatic amine oxides. For example, hydrogen peroxide is too weak of an oxidizing agent to be effective. Thus, the use of peracids is dictated. Shaw et al., J. Am. Chem. Soc., 72, 4362 (1950) oxidizes 2-bromopyridine with perbenzoic acid or peracetic acid to form the corresponding aromatic amine oxide. Reported reaction times amount to several days with 60-70 percent yields resulting. Finger and Starr, J. Org. Chem., 24, 2674 (1959) show the preparation of 2-chloropyridine-N-oxide by reacting 2-chloropyridine with glacial acetic acid and 40 percent peracetic acid. An 80 percent yield is reported after about 22 hours reaction time.
Semenoff, U.S. Pat. No. 2,745,826, oxidizes 2-bromopyridine with 40 percent peracetic acid to obtain a 71 percent yield of the desired amine oxide product in about 4 hours. Ochiai, J. Org. Chem., 18, 534 (1953) also shows the use of glacial acetic acid and 35 percent aqueous hydrogen peroxide to form pyridine-N-oxide in about 96 percent yield after 12 hours reaction. Quinoline-N-oxide was similarly prepared using 29 percent aqueous hydrogen peroxide to obtain the desired product in about 92 percent yield after an unspecified reaction time.
Common in these prior proposals in the preparation of aromatic amine oxides only after extended reaction times and then generally to no more than about a 60-80 percent yield. Also, cumbersome and time consuming purification of the resulting products are reported. While such yields may appear to be satisfactory at first glance, it must be remembered that the aromatic amine reactants, eg. 2-chloropyridine, often range in price to several dollars per pound. The high cost of such reactants necessarily dictates that a very efficient commercial process be employed in order to provide a commercially, economically viable process. Moreover, a simple method for recovery of the desired product also is necessary. Even if these objectives were accomplished, economics of the marketplace also dictate that reaction times be reduced substantially in order to make the process more commercially viable and attractive. The present invention provides an answer to the commercial need for the preparation of aromatic amine oxides rapidly, at moderate reaction conditions, and in essentially stoichiometric yields.